I have 4 Geforce GTX 1080. Nvidia 378.66 driver Windows 10 64bit. Blender v2.78b
Blender User Preferences -> System -> Cycles Compute Device -> None
Cycles render does not recognize my 4 Geforce GPU boards. I worked in previous Nvidia and Blender versions.
Therefore the GPU Compute under render settings is also grayed out.
I see that my original post has been marked as a duplicate but the solution to post GPU compute grayed out is not applicable to mine.
I have no CUDA option in User Preferences -> System -> Cycles Compute Device It says "None" there